Guard electrode apparatus for boom of insulated aerial device

ABSTRACT

Apparatus for measuring leakage current passing through the insulative section of the boom of an aerial device for lifting workmen to an elevated height wherein is provided a conductive element on the outer surface of the insulative section for collecting leakage current passing therethrough, and a grounded conductive shield disposed in electrically insulated, overlying relationship to the conductive element to prevent extraneous ambient electrical currents from reaching the conductive element and being measured.

United States Patent [1 1 Staley GUARD ELECTRODE APPARATUS FOR BOOM OFINSULATED AERIAL DEVICE Inventor: Jerry A. Staley, Centralia, Mo.

Assignee: A. B. Chance Company, Centralia, Mo.

Filed: Apr. 13, 1972 Appl, No; 243,559

US. Cl. 324/54, 174/5 80 Int. Cl. G01! 31/02, G01r 31/12 Field of Search324/54, 72;

174/5 SG, 11 BH [56] References Cited UNITED STATES PATENTS 3,320,5245/1967 Miller 324/54 3,054,049 9/1962 Chance 324/54 Oct. 30, 19732,170,813 8/1939 Doble et a1. 324/54 3,449,668 6/1969 Blackwell et al.

3,612,994 10/1971 Hooper 324/54 Primary Examiner-Gerard R. StreckerAttorney-Schmidt, Johnson, l-lovey & Williams [57] ABSTRACT Apparatusfor measuring leakage current passing through the insulative section ofthe boom of an aerial device for lifting workmen to an elevated heightwherein is provided a conductive element on the outer surface of theinsulative section for collecting leakage current passing therethrough,and a grounded conductive shield disposed in electrically insulated,overlying relationship to the conductive element to prevent extraneousambient electrical currents from reaching the conductive element andbeing measured.

7 Claims, 4 Drawing Figures boom members has an insulated section. Morespecifically, the invention pertains to means for measuring leakagecurrent passing through the insulative section .of the boom member. v V

An articulated aerial device of the class described usually incorporatesan insulative section in one of the boom members in order toelectrically insulate the working station at the upper end of the boomfrom the vehicle upon which the aerial device is mounted. The insulativesection permits the working station to be brought near high voltagepower lines or the like while maintaining maximum safety for workmen.This is especially required for live-line bare-hand work. It isimportant that the insulative characteristics of the boom be frequentlymonitored to assure that contamination or other conditions have notformed a conductive path along the boom sufficient to present adangerous electrical discharge path from the working station to thevehicle.

Various methods of detecting leakage current through the insulativesection of the aerial boom have been devised, and the most reliableindication of the insulative condition of the boom is determined byplacing a leakage current pickup conductor near the lower end of theinsulative portion of the boom and measuring the current collectedthereby. The current measured through the pickup conductor band isindependent of the normal working position of the aerial device orvehicle. The pickup band does, however, collect substantial extraneouselectrical currents from the ambient atmosphere generated by overheadpower line or other sources. These extraneous currents will normallyconstitute a substantial portion of the current collected and measuredin view of the very small mangitude of current which leaks through theinsulative section, and such ambient electrical currents may varysubstantially in value with changes in humidity and other atmosphericconditions. The resulting insensitivity to the actual current leakagethrough the insulative section of the boom makes it particularlydifficult to detect any changes in the insulative integrity of the boom,since the boom may be in a different locale and thereby subjected todiffering wheather conditions between readings of the current leakage.While the monitoring of current leakage may be performed underlaboratory conditions wherein the external-environmental factors may becontrolled, it is important for safety reasons that the boom be testedunder actual working conditions where extraneous ambient electricalcurrents may well give rise to false readings and prevent determinationof the actual electrical conditions of the boom, and make itparticularly difficult to determine any significant changes in theinsulative condition of the boom.

It is a broad object of the present invention to provide guard electrodeapparatus for collecting extraneous ambient electrical currents toprevent the latter from reaching the conductive element which collectsleakage current passing through an insulative section of an aerial boomsuch as those of the articulated type so that the insulative integrityof the boom may be reliably measured.

It is another object of the invention to provide apparatus formonitoring current leakage through the insulative boom which includes aconductive element on the outer surface of the insulative section of theboom to collectleakage current passing therethrough, detecting means formeasuring current collected by the conductive element, and guard meansfor collecting extraneous ambient electrical currents to prevent thelatter from reaching the conductive element so that said apparatusmeasures substantially only the boom leakage 'current.

Another object of the invention is to provide a conductive element whichextends transversely around the outer surface of the boom to collect agreat portion of the current leaking therethrough, and a guard conductorsurrounding the band and extending past the opposed side margins thereofand electrically insulated therefrom in'order to effectively collectextraneous ambient electrical currents and prevent the latter fromreaching the conductive band so that the magnitude of current collectedby the band is a sensitive indicator of the current leakage through aninsulated aerial boom.

These and other objects and advantages of the present invention arespecifically set forth in or will be come apparent from the followingdetailed description of a preferred embodiment of the invention whenread in conjunction with the accompanying drawing, wherein:

FIG. 1 is a side elevation of an insulated aerial device mounted upon avehicle set up in testing condition to monitor current leakage throughthe insulative boom;

FIG. 2 is an enlarged, partial longitudinal crosssectional view of theinsulative section of the boom and guard electrode apparatus ascontemplated by the present invention with the leakage monitoringcircuitry shown schematically;

FIG. 3 is a transverse cross-sectional view taken along lines 3-3 ofFIG. 2 with portions broken away to reveal details of construction; and

FIG. 4 is a side plan view of guard electrode apparatus of the presentinvention.

FIG. 1 depicts a utility vehicle 10 upon which is mounted an articulatedaerial device generally referred to by the numeral 12, which has anupper boom section 14 composed of electrically insulative material suchas glass reinforced plastic. The lower end of insulative boom section 14is afi'ixed within metallic boot l6, and at the upper end of section 14there is provided a working station which may include one or moreworkman baskets 18. The aerial device 12 is pivotal upon its turret base20, and the lower boom section 22, insulative section 14 and the basket18 are individually adjustable by hydraulic actuators in order that theupper end of the boom may be located as desired, such as adjacent anoverhead high voltage power line 24. The insulative section 14 is ofhollow, rectangular cross section presenting an internal space 26 forcarrying electrically insulated hydraulic conduits, control linkage orother appurtenances from the lower boom section 22 to the upper end ofinsulative section 14.

Conductive elements in'the form of thin bands 28 and 30 of metallic,electrically conductive foil, respectively, extend transversely aroundin engagement with the inner and outer surfaces 32 and 34 of theinsulative section of the boom. Conductive bands 28 and 30 are locatedat the lower end of insulative section 14 in adjacent relationship tothe conductive boot 16 of the aerial device, and are slightly offsetlongitudinally relative to one another. The conductive bands areelectrically interconnected by shielded conductor cable 36 with currentdetecting means in the form of a microammeter 38 that is positioned onthe vehicle at a convenientand a'ccessible location. The shielded cableor cables 36 extend from the vehicle proper through the internal boomspace 26 up to the conductive bands and extends transversely through theinsulative boom section 14 for interconnection with the outer conductiveband 30.

Guard means 40 extend transversely around and are removably secured atthe outer surface 34 of the insula, tive section in surrounding,covering relationship to the outer conductive band 30. The guard meansis defined by half sections 42 and 44, each of which includes anintermediate, generally U-shaped metallic band or guard conductor 46which is encapsulated in a U- shaped, outer member 48 of aweather-resistant, insulative synthetic resin material and therebypresenting a layer of insulation between boom section 14 and theinnermost surfaces of respective metallic bands 46. As apparent from HO.3, the ends '52 of metallic band 46 project outwardly from opposed endstretches of member 48 and are bent outwardly at right angles torespective main legs thereof to facilitate interconnection of opposedsections 42 and 44. The ends 52 of sections 42 and 44 have suitableopenings therein for receiving machine screws 54 or the likethat-releasably, mechanically and electrically interconnect the adjacentexposed ends of the guard conductors'46 associated with each halfsection. A conductive wire 56 electrically interconnects theguardconductor 46 with boot 16 which, in turn, is electricallyinterconnected with the body of vehicle 10. The inner surface 50 of eachmember 48 is provided with an inwardly facing notch 51 therein receivinga conductive band 30 and of a greater width than the latter. in thismanner, sections 42 and 44 are maintained out of contacting relationshipwith band 30.

Guard conductor 46 is substantially wider than outer conductive band 30and extends past the opposed side margins 58 of the latter. The guardmeans 40 thereby surroundsand substantially encloses the outerconductive band 30 without contacting the latter, it being noted thatthe exposed proximal stretches 52 of the guard conductors of halfsections 42 and 44 are located quite close, leaving only sufficient gapfor tolerance adjustment to assure a tight fit of guard means 40 on thetially completely isolated from the surrounding atmosphere.

Tests have shown that the plastic encapsulate member 48 should be ofdimensions presenting an inner segment maintaining metallic band 46 ineach half section 42 and 44 inspaced relationship from the outer surfaceof boom section14 a distance of about five-sixteenths inch. This spacingwas found to give optimum screening against extraneous ambientelectrical currents. Closer spacing resulted in pickup of strayelectrical currents from the boom, while wider spacing of members 46from boom section 14 allowed ambient electrical currents to affect theammeter readings. Similarly,

' viewing FIG. 2, the optimum distance between the lefthand margin ofband 20 to the right-hand edge of each member 46 should be about 2%inches for the same reasons explained with respect to the spacingbetween members 46 and boom section 14, where band 30 has a nominalwidth of three-fourths inch and each member 46 is 3 inches wide.

. boom, so that the outer conductive band is substan- I The apparatus isset up for field testing of the current leakage through the insulativesection l4by first positioning aerial device 12 with its outer enddisposed on the ground directly below' the overhead line 24. The vehiclebody is solidly grounded through an electrical grounding cable 62, andthe microammeter 38 is securely grounded to the vehicle body andinterconnected with the conductive bands 28 and 30. It will be notedthat guard conductor 46 is grounded through conductor 56, boot 16 andthe vehicle body. A conductive member such as a heavy copper cable 64 isbolted to the conductive bucket mounting bracket 66, and the aerial boomis then lifted until contacting rod 64 is brought into engagement withthe overhead poawer line 24. Measurement of boom leakage current incontrolled laboratory conditions is accomplished in a similar mannerwith the exception that the upper end of the insulative section of boom14 is energized directly by a high voltage source, KV or greater. Themicroammeter 38 is then monitored to determine the leakage currentspassing through the insulative section 14 along the outer surfacethereof and through its internal space 26.

The boom leakage currents are primarily capacitive in nature and areschematically depicted in FIG. 2 by C1 and C2 which respectivelyrepresent the leakage current through the internal space 26 of the boomand along the outer surface 34 of boom section 14. [t is important thatleakage current along outer surface 34 be monitored as such may changedrastically with time due to the affects of weathering, contamination,moisture or insulation damage. Most commonly, for instance,contamination on the outer surface 34 will collect moisture that mayform a film capable of conduct-- ing high current flow across insulativesection 14. The ground means 40, while surrounding conductive band 30 inoverlying relationship, does not interfere with the collection ofleakage currents along outer surface 34 by band 30 in view of thepositioning of guard conductor 46 in spaced, insulated relationship fromboom surface 34. The leakage current along insulative section 14 isrelatively small in magnitude however, on the order of approximately 100microamperes or less, and yet such current must be measured withsufficient accuracy so as to reliably reflect small changes in themagnitude of the current leakage so that boom cleaning or otherpreventative measures may be taken prior to formation of dangerousconductive conditions along the boom.

Extraneous ambient electrical currents, illustrated schematically by C3in FIG. 2, which emanate from the overhead power line 24 and othersources and pass di rectly through the air to the outer conductive band30 are sufficiently large in magnitude so as to mask over the actualleakage currents C1, C2 of the boom and render the measurements ofmicroammeter 38 relatively meaningless. The guard means 40, however, ispositioned in shielding, overlying relationship to outer conductive band30 so as to collect these extraneous ambient-electrical currents C3prior to their reaching conductive band 30. Guard conductor 46 isgrounded through boot 16 so as to shunt these extraneous ambientelectrical currents to ground so that the microammeter 38 detectssubstantially only the boom leakage currents C1 and C2.

While the actual size and shape of the guard conductor 46 and itsposition relative tov conductive band 30 may vary depending upon theconfiguration of band 30 itself, optimal shielding of the conductiveband 30 from the extraneous ambient electrical current C3 results bymaking the guard conductor 46 wider than conductive band 30 andpositioning the guard conductor so as to extend past the opposed sidemargins 58 of band 30, along with locating the guard conductor aboveband 30 in spaced, surrounding relationship thereto. Such arrangementpermits maximum shielding of the conductive band 30 from extraneouselectrical currents C3 while leaving a conductive path along the outersurface 34 for the boom leakage current C2 to be collected by band 30.The 'outer protective layer of guard means 40 serves as an effectivemeans for protecting said guard conductor from mechanical abrasion anddamage by external objects such as tree limbs.

Multiple, concentric guard conductors may be incorporated within a guardmeans if necessary in order to further enhance the shielding of outerconductive band from the extraneous ambient currents, While in thepreferred embodiment illustrated there is provided an insulative supportstrap 48 which directly engages the outer surface 34, the guardconductor 46 may be otherwise supported in insulated, spacedrelationship to band 30 such as by directly securing guard conductor 46to boot 16 in overlying but spaced disposition from band 30.

The guard conductor has also been found in practice to guard the innerpickup band 28 from ambient electrical currents. The proximal locationof guard conductor 46 to band 28 collects and reduces the flow of ambient electrical currents that would otherwise reach band 28. In similarfashion the guard conductor protects a conductive band disposedtransversely around the outside surface of an insulative section of saidboom for collecting leakage current passing through said section;

detecting means electrically connected with said band for measuring saidboom leakage current;

at least one guard conductor disposed in spaced, surrounding, coveringrelationship to said band, said guard conductor being wider than saidconductive band and extending beyond the opposed side marginal edgesthereof;

an insulative support carried by the boom in supporting relationship tosaid guard conductor to maintain the latter electrically insulated fromsaid band, said support including a layer of synthetic resinous materialdisposed between the guard conductor and the band and at least partiallyincapsulating the guard conductor to protect the latter from mechanicalabrasion and damage by external objects; and

ground means connected to said guard conductor for electricallygrounding the latter whereby said detecting means measures substantiallyonly said boom leakage current.

2. Apparatus as set forth in claim 1, wherein said boom is hollow, andfurther includes a second conductive band disposed transversely aboutthe inside surface of said boom for collecting leakage current passingthrough said boom, said detecting means being electrically connectedwith said first and second bands respectively for measuring said boomleakage current.

3. Apparatus as set forth in claim 1, wherein said device is mounted onan electrically grounded vehicle body, said ground means electricallyconnecting said guard conductor with the vehicle body whereby to groundsaid guard means.

4. Apparatus as set forth in claim 1, wherein said support layer has asegment between the guard conductor and boom of sufficient thickness tomaintain the distance therebetween at about five-sixteenths inch.

5. Apparatus as set forth in claim 1, wherein said support layer has agroove on the inner surface thereof which is configured and arrangedsuch that the layer does not contact said band.

6. Apparatus as set forth in claim 1, wherein said guard conductorpartially incapsulated by synthetic resinous material is divided intotwo U-shaped sections, there being means for releasably, mechanicallyand electrically interconnecting the conductors of said two sections andfor securing the latter on the outside surface of said boom.

7. Guard apparatus for a conductive band disposed transversely aroundthe outside surface of the insulative section of a boom of an aerialdevice to collect leakage current passing through said section, saidapparatus including:

a guard conductor disposed externally of said band in surrounding,covering, electrically insulated relation-ship thereto, said guardconductor being wider than said band and extending past the opposed sidemarginal edges thereof and being operable to collect extraneous ambientelectrical currents not passing through said boom to prevent the samefrom reaching said band;

an insulative support carried by the boom in supporting relationship tosaid guard conductor to maintain the latter electrically insulated fromsaid band, said support including a layer of synthetic resinous materialdisposed between the guard conductor and the band and at least partiallyincapsulating the guard conductor to protect the latter from mechanicalabrasion and damage by external objects; and

ground means connected to said guard conductor for electricallygrounding the latter whereby said detecting means measures substantiallyonly said boom leakage current.

1. Apparatus for monitoring current leakage through the electricallyinsulative section of a boom of an aerial device, comprising: aconductive band disposed transversely around the outside surface of aninsulative section of said boom for collecting leakage current passingthrough said section; detecting means electrically connected with saidband for measuring said boom leakage current; at least one guardconductor disposed in spaced, surrounding, covering relationship to saidband, said guard conductor being wider than said conductive band andextending beyond the opposed side marginal edges thereof; an insulativesupport carried by the boom in supporting relationship to said guardconductor to maintain the latter electrically insulated from said band,said support including a layer of synthetic resinous material disposedbetween the guard conductor and the band and at least partiallyincapsulating the guard conductor to protect the latter from mechanicalabrasion and damage by external objects; and ground means connected tosaid guard conductor for electrically grounding the latter whereby saiddetecting means measures substantially only said boom leakage current.2. Apparatus as set forth in claim 1, wherein said boom is hollow, andfurther includes a second conductive band disposed transversely aboutthe inside surface of said boom for collecting leakage current passingthrough said boom, said detecting means being electrically connectedwith said first and second bands respectively for measuring said boomleakage current.
 3. Apparatus as set forth in claim 1, wherein saiddevice is mounted on an electrically grounded vehicle body, said groundmeans electrically connecting said guard conductor with the vehicle bodywhereby to ground said guard means.
 4. Apparatus as set forth in claim1, wherein said support layer has a segment between the guard conductorand boom of sufficient thickness to maintain the distance therebetweenat about five-sixteenths inch.
 5. Apparatus as set forth in claim 1,wherein said support layer has a groove on the inner surface thereofwhich is configured and arranged such that the layer does not contactsaid band.
 6. Apparatus as set forth in claim 1, wherein said guardconductor partially incapsulated by synthetic resinous material isdivided into two U-shaped sections, there being means for releasably,mechanically and electrically interconnecting the conductors of said twosections and for securing the latter on the outside surface of saidboom.
 7. Guard apparatus for a conductive band disposed transverselyaround the outside surface of the insulative section of a boom of anaerial device to collect leakage current passing through said section,said apparatus including: a guard conductor disposed externally of saidband in surrounding, covering, electrically insulated relationshipthereto, said guard conductor being wider than said band and extendingpast the opposed side marginal edges thereof and being operable tocollect extraneous ambient electrical currents not passing through saidboom to prevent tHe same from reaching said band; an insulative supportcarried by the boom in supporting relationship to said guard conductorto maintain the latter electrically insulated from said band, saidsupport including a layer of synthetic resinous material disposedbetween the guard conductor and the band and at least partiallyincapsulating the guard conductor to protect the latter from mechanicalabrasion and damage by external objects; and ground means connected tosaid guard conductor for electrically grounding the latter whereby saiddetecting means measures substantially only said boom leakage current.